Two series of model hemes will be synthesized and their physical properties investigated. Physical properties to be studied include redox potentials and rates of electron exchange, molecular structures, thermodynamics and kinetics of axial ligation, uv-visible, nmr, esr, mcr, resonance Raman, and Mossbauer spectra. The two series of compounds proposed for study are: 1) pairs of six-coordinate iron porphyrins in which the axial ligand planes are forced to be either mutually parallel or perpendicular, and 2) a series of unsymmetrical pyrrole-substituted synthetic porphyrin complexes. The purpose of these studies is to determine quantitatively the effect of axial ligand plane alignment and unsymmetrical pyrrole substitution on the energies of the metal d and porphyrin pi-orbitals. Their splitting is expected to explain the variation in redox potentials, electron transfer rates, nmr contact shifts, esr g-values, and other properties among the cytochromes b, of which the proposed molecules are direct models. Erythrocyte cytochrome b5 will be isolated from bovine blood and investigated by nmr and electrochemical techinques in the absence and presence of additives such as acid, drugs, and urea to see if they bring about heme rotation, and if so, whether this heme rotation brings about a change in the redox potential of this cytochrome b5. These studies are aimed at determining whether heme rotation affects the ability of erythrocyte cytochrome b5 to reduce methemoglobin, and if so, what substances and conditions bring about heme rotation.